A friend of mine told me about the Pilot Frixion point eraseable gel pen. A group of my friends tried it and it has a very fine mark that completely disappears with heat! Awesome!! Completely gone! Even black. It is available at Office Max and other places and comes in lots of colors - the main ones being black, blue and red.

I tried it and it took 2 minutes in the freezer to reappear and five minutes in the refrigerator to reappear. Thats 40 degrees.
I'd be afraid of it coming back transporting it in the winter. Someone said they washed it then put it in the freezer and it didn't reappear but I haven't tried that yet.